1. Field of the Invention
The present invention relates to a beam modifying device, a lithographic projection apparatus, a method of treating a beam and a device manufacturing method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
As will be explained in more detail below, a lithographic apparatus generates a patterned beam to apply a desired pattern on a substrate. A radiation system is provided, including a source, an illuminator, and a beam delivery system (if required), to generate a beam. In order to get optimal results, the beam should be well defined, having constant and predictable properties. The beam should be symmetrical, i.e. in shape, intensity distribution and angular distribution.
However, in practice, no perfectly symmetrical and/or uniform radiation beams are obtainable. For instance, the Cymer XLA-165 laser, is known to produce a beam having a constant contour (due to a rectangular diaphragm inside the laser), but having a fluctuating and asymmetric intensity distribution of the beam cross-section. This results in unstable measurements of beam positioning and pointing. It is known that high-power lasers in general do not have a constant and symmetric intensity profile: the profile changes in time, e.g. due to laser refills or gas heating or gas burn-up.
In many cases the beam must be kept constant in position to below 0.5 mm and in pointing to below 30 μrad at the illuminator entrance so that the imaging results are not negatively impacted.